Compact ranges are used to illuminate test objects with a planar electromagnetic wave front in a minimum of space. Such ranges typically may be contained within an anechoic chamber or a room lined with absorber and thus eliminate sources of stray reflections found in outdoor ranges. In addition, the operation of such indoor ranges is not affected by the weather. Additional advantages include increased security resulting from conducting measurements of sensitive test objects indoors rather than outdoors and efficiencies achieved in instrumenting, setting up and adjusting the illuminating antenna and the test object in near-field conditions rather than requiring range operators to make numerous trips between the transmit site and the test site over the far-field distance typically required in outdoor ranges.
Compact ranges typically utilize a feed positioned at the focal point of a paraboloidal reflector to allow the reflector to reflect the spherical wave radiated by the feed as a planar wave. Such compact ranges are disclosed in U.S. Pat. No. 3,302,205 issued Jan. 31, 1967 to R. C. Johnson, which patent is incorporated herein by reference. Other secondary reflectors may also be used in addition to the paraboloidal reflector.
The Johnson patent shows the use of only a small portion of the range reflector to produce a planar wave in order to avoid aberrations caused by surface and edge currents. More recent techniques, however, allow a larger portion of the range reflector to be utilized. Such techniques include rolling, blending, and serrating the edges of the reflector. Nevertheless, the "quiet zone", which is the region of space on the range in which a uniform plane wave is produced, is limited to a cross section whose size is small relative to the total reflector area. For instance, a typical quiet zone may be six feet by four feet in cross-section for a ten foot radius reflector, or twenty-four square feet for a greater than three hundred square foot reflector.
Compact ranges, including offset paraboloidal reflector based compact ranges, also suffer from amplitude taper in the quiet zone field. This amplitude taper is caused by (1) differential space loss or spreading loss from the feed to different points on the reflector and (2) the amplitude roll-off of the feed pattern. Although this taper is an inherent feature of compact ranges, optimized feed designs can minimize it.